upgrade from highest plan

My highest plan does not have any upgrades set up but still the users on that plan can "upgrade to the lower plan. How come?

I don't know how it could happen. In this case, please submit a support ticket sending us:

1. Super admin account of your site

2. A subscriber account which has that upgrade issue

3. Link to the page which we can see that upgrade option

We will check to see what's wrong and get it corrected

Tuan